Multiple arcing gap



Nov. 1, 1938. H. 1.. RORDEN 2,135,352

MULTIPLE ARC ING GAP Filed 001;. 25, 1935 INVENTOR ham/d L. fiar'den BYPatented Nov. 1, 1938 UNITED STATES PATENT OFFICE MULTIPLE ABCING GAPApplication October 25, 1335, Serial No. 48,753

Claims.

This invention relates to protective gaps for electrical apparatus, andhas for one of its ob- Jects the provision of a gap having substantiallyconstant characteristics and a short time lag,

especially for high transient voltages.

A further object of the invention is to provide a multiple gap in whichthe division of voltage between a plurality of gaps in series shallremain substantially constant notwithstanding exposure to weather andair laden with foreign material.

A further object of the invention is to provide protective gap apparatusfor high voltages which shall have a short time lag and substantiallyconstant characteristics.

A further object of the invention is to provide a device of the classnamed which shall be of improved construction and operation.

Other objects and advantages will appear from the following description.

The invention is exemplified by the combination and arrangement of partsshown in the accompanying drawing and described in the followingspecification, and it is more particularly pointed out in the appendedclaims.

In the drawing:

Fig. 1 is an elevation of a multiple gap showing one embodiment of thepresent invention.

Fig. 2 is a circut diagram showing the relation of the variouselectrical quantities of the device shown in Fig. 1.

In the protection of electrical apparatus from lightning, it hasheretofore been the practice to provide an arcing gap in parallel withthe apparatus for the purpose of diverting the surge due to voltageinduced by lightning, or to a direct stroke, so as to protect theapparatus from injury due to the surge. Heretofore, it has generallybeen assumed that if the gap is set so as to flash over at a lowervoltage than the associated apparatus, the apparatus would be protected.There is, however, another factor besides voltage which must be takeninto consideration and that is the flashover time lag of the gap and theapparatus. By time lag in this connection is meant the time whichelapses between the impression of a voltage of sufilcient magnitude toproduce fiashover, and the discharge. This time lag varies with thenature of the gap or of the apparatus. A needle or rod gap, iorinstance, although it will fiashover at a lower voltage for a givenspacing than a sphere gap'of proper dimensions, has a much longer timelag than does the sphere gap. This is due to the fact that in the caseof a needle or rod gap ionization between the electrodes proceedsprogressively from each electrode toward (Cl- PIS-30 the other, andfiashover occurs when the corona, built up from the two electrodes,spans the space between the two. In the sphere gap, on the other hand,ionization takes place across the entire gap almost simultaneously withthe impression of a voltage sufficient to cause fiashover. Thus, if aneedle gap and a sphere gap of the same spacing were subjected to aslowly increasing voltage, the needle gap would fiashover at a muchlower voltage than the sphere gap, but if the two were subjected to awave of steep front of sufficient volt age to fiashover the sphere gap,the sphere gap would go ahead of the needle gap because of thedifference in time lag characteristics of the two gaps.

It is apparent, therefore, that for the protection of electricalapparatus, the quick action of the sphere gap is to be preferred to theslow action of the needle or rod gap, but there are other disadvantagesin the use of a sphere gap. It must be set closer for a given voltagethan a rod gap, and if the surfaces of the spheres for any reason do notremain substantially true and smooth, they may take on thecharacteristics of a needle gap and consequently fiashover at a muchlower voltage than that for which they were originally set and thuscause unnecessary and troublesome interruptions at much lower voltagesthan that for which the apparatus requires protection. A drop of wateron the surface of one of the spheres, or an insect or any other foreignmatter, may change the nature of the sphere gap so as to interfere withits true operation.

In order to avoid the defects of both the sphere gap and the rod gap, ithas been proposed to use a gap having modified terminals approachingthose of a rod gap and to control the electrostatic field about thedischarge terminals by insulated flux controls so as to produce a fieldsimilar to that of the sphere gap, thus giving the gap the short timelag characteristics of a sphere gap while avoiding the effects of slightvariations in the surface of the discharge terminals. Such a gap isshown in Patent No. 2,011,136, granted Aug. 13, 1935 to Arthur 0. Austinand assigned to The Ohio Brass Company, of Mansfield, Ohio. This gap hasbeen found to work efllciently for voltages up to approximately 100kilovolts but above that voltage it is necessary, in order to make thedevice maintain constant characteristics, to use porcelain parts of suchlarge proportions that it becomes expensive and consequently not wellsuited for commercial operation.

The present invention provides means for overcoming this difliculty andconsists broadly in the use of two gaps similar to that of the Austinpatent, connected in series with means for maintaining a uniformdivision of voltage between the series gaps. Heretofore it has beenfound that when gaps were connected in series, the total voltage acrossthe series would be divided between the gaps only so long as theintermediate terminals were insulated from ground by practicallyinfinite impedance. It has been found, however, practically impossibleto maintain such impedance under ordinary conditions of operation. Anydirt or moisture on the support for the intermediate terminal causessuflicient conductivity to ground to bring the intermediate terminal toa potential approaching ground potential with the result that thefiashover voltage of the device is not the sum of the two gaps but theflashover voltage of the gap connected to the high potential, since theintermediate terminal approaches ground potential. After fiashoveroccurs between the high potential terminal and the intermediateterminal, the intermediate terminal at once takes on the voltage of thehigh potential terminal and flashover readily occurs between theintermediate terminal and the ground terminal. The present inventionprovides means for maintaining the distribution of voltage between theseveral gaps, notwithstanding variation in the impedance of the circuitto ground from the intermediate terminal.

In the embodiment of the invention shown in Fig. l, the numerals l0 andII indicate the terminals of an arcing gap having insulated flux controlmembers l2 electrically connected therewith in the manner more fullydescribed in the Austin patent referred to above. The terminal I0 issupported by an adjusting rod 13 provided with means It by which it isconnected to the high potential transmission line. The rod i3 issupported by insulators l5 having relatively high capacitance and theseinsulators 15 are in turn supported by a stack of insulators 16 whichrest upon a grounded support IT. The gap terminal i is carried by anadjusting rod I8 which is mounted on a pedestal I8 carried by insulators20 similar to insulators IS, the insulators 20 also resting upon thegrounded support IT. A second gap formed of gap terminals 2| and 22 andhaving electrostatic control members 23 is connected in series with thefirst gap, the terminal 22 being substantially at ground potential aswill be more fully explained. A conducting tie or jumper 25 connects theupper ends of the insulator stacks I6 and 20 so that these stacks areconnected in parallel and provide a capacitance between the intermediateconductor terminal member I! and ground H.

The relation of the parts will be understood more readily from Fig. 2 inwhich the two gaps are designated by the numerals A and B respectively,and the capacitance provided by the insulators I5 is designated by theletter C, and the combined capacitance of the two stacks of insulatorsI5 and 20 is designated by the letter D. From this diagram, it will beseen that an impedance path from the high potential line to ground isformed by the two capacitances C and D in series. It will also beapparent that the voltage drop for an alternating or impulse dischargeover this impedance path will be divided between the two capacitances Cand D in proportion to the impedances of the respective capacitors. Thiswill be true irrespective of the wave front or frequency of the impulseor alternating current. If the capacitance of the members C and D islarge enough to pass a current of sumcient energy, the division ofvoltage between the two parts of the circuit will remain substantiallyconstant, notwithstanding any slight variation in the capacitance of theinsulators l5, l6 and 20, due to foreign matter on their surfaces. Thepotential of the intermediate terminal or the voltage gradient betweenthe terminals Ill and 22 is, in this way, determined by the division ofimpedance between the condensers C and D so that the ilashover voltageof the two gaps A and B in series may be approximately the sum of theflashover voltages of the two gaps taken separately. In this way, anydesired total flashover voltage may be built up by a series of smallergaps in which the time lag characteristics may be accurately determinedwithout the use of excessively large parts.

Generally, it is desirable to procure a substantially equal division ofvoltage between the several gaps in series, to secure the maximumbenefit of the invention and make possible the use of uniform fluxcontrols of minimum size for all gaps. However, beneficial results maybe obtained so long as the fixed relative impedances of the condensers Cand D remain the dominant factor in determining the voltage distributionor the discharge circuit. It has been found by test: that for voltagesfor which the device is intended, namely, all commercial voltages abovekv., a capacitance of approximately seventy-five micro-microfarads forthe condenser C and thirty micro-microfarads for the condenser D givesvery satisfactory results and that the leakage conductivity of thecondensers may be varied by moisture or other contamination to values upto 25 percent of their capacitance conductivity without seriouslyinterfering with the operation of the device. If, under severeconditions, it is found that the capacitance values given above are notsuiilcient to maintain eflicient distribution of voltage between thegaps, the capacitance of the insulators may be increased but it isbelieved that the values given above will be found suitable for mostconditions.

The flu)! control members I2 and 23 provide a distribution of theelectrostatic field about the gap terminals which will give a short timelag to the gap which is maintained even with a plurality of gaps inseries. This is important in the protection of various forms ofelectrical apparatus and especially in the protection of transformers orother apparatus in which the parts are immersed in oil. Apparatus inwhich the discharge terminals are covered with oil has a short time lagdischarge characteristic similar to that of sphere gaps because of thesuppression of the corona due to the oil about the electrode surfaces.In the protection of this kind of apparatus, it is therefore of utmostimportance to have a gap with a short time lag characteristic asotherwise the apparatus is apt to fail ahead of the gap flashover.

The invention thus far described has been found to be well adapted forthe protection of all kinds of electrical apparatus. In installations,however, where a lightning discharge is apt to be followed by a powerarc and where the overload cut-outs used in connection with the systemare not sufliciently rapid to clear the line without causinginterruptions, it may be desirable to provide means in connection withthe gap for breaking the power arc. To provide for this operation theterminal 22, instead of being directly grounded, is supported bycondensers 28.

These condensers 26 have sufficiently high capacitances so that when asteep wave impulse is impressed upon the conductor, the terminal 22 willbe substantially at ground potential so far as the steep wave impulse isconcerned and the two gaps will flashover under impulse substantially vthe same as if the electrode were directly connected to ground through aconductor. A capacity of 300 micro-microfarads has been found suitablefor the condenser 26. As soon as flashover occurs, the electrode 22 willbe raised substantially to linepotential and arcing will then take placethrough an arc interrupter 21 to ground. The interrupter 21 may be atubular member of insulating material, such as hard fibre, havingterminals at the opposite ends thereof for directing a discharge throughthe interior of the tube. Such a device will clear the power are andleave the apparatus ready for further operation. Of course a fuse orother disconnecting device could be connected between terminal 22 andground and would clear the line 01 the power arc, but a tubularinterrupter of the nature described has the advantage that it does notrequire replacement after an operation and will therefore clear the arefor multiple discharges which are very common in impulses produced bylightning. A tubular clearing device has the further advantage that itdoes not require replacement after an interruption.

I claim:

1. The combination with a high potential transmission line, ofprotective means for apparatus connected to said line, said protectivemeans comprising a plurality of arcing gaps connected in series in acircuit between said line and ground, each of said gaps having fluxcontrols associated with the terminals thereof for imparting a shorttime lag characteristic to said gaps, normally open circuit means inseries with said gaps for interrupting a power arc following an impulsedischarge over said gaps, a condenser in parallel with said power areinterrupting means to facilitate discharge across said gaps and aplurality of condensers in series with each other in a circuit inparallel with said first named circuit, each of said condensers being inshunt with a portion of said first named circuit including one of saidgaps.

2. The combination with a high potential transmission line, of means forprotecting electrical apparatus connected with said line, said meanscomprising a pair of arcing gaps and a condenser connected in seriesbetween said line and ground, means for controlling the division ofvoltage between said gaps, and normally open circuit means in parallelwith said condenser for interrupting a power arc following a dischargeover said gaps, said open circuit means being adapted to flash over whensaid gaps have flashed over to impress on said normally open circuitmeans the excess voltage on said line.

3. The combination with a high potential transmission line, of means forprotecting electrical apparatus, connected with said line, from highpotential surges, said protecting means comprising a pair of arcing gapsand a condenser connected in series between said line and ground, atubular member in parallel with said condenser for interrupting powerarcs following impulse discharges over said arcing gaps, and animpedance path comprising a plurality of condensers connected betweensaid line and ground, the intermediate electrode between said arcinggaps being connected at an intermediate point in said impedance path tocontrol the division of voltage between said arcing gaps.

4. In combination a high potential electrode having a discharge terminalthereon, a low potential electrode having a discharge terminal thereon,an intermediate electrode having a pair of discharge terminals thereoncooperating respectively with the terminals on said high potential andsaid low potential electrodes, capacitance means for supporting each ofsaidelectrodes and for insulating said electrodes from ground, a portionof the capacitance of said means being connected in parallel with thegap between said high potential and said intermediate electrodes, a lessportion of said capacitance means being connected in parallel with thegap between said intermediate electrode and said low potentialelectrode, the capacitance between said low potential electrode andground being several times the capacitance in parallel with either ofsaid gaps, an arc extinguishing tube in parallel with the capacitancebetween said low potential capacitance and ground, and means fordirecting a discharge arc through said are extinguishing tube from saidlow potential electrode to ground.

HAROLD L. RORDEN.

